I have had the same problems with IPv6 and Gmail. As I also admin G.Suite I have access to their paid support. I did your exact follow up - politely and persis

I have had the same problems with IPv6 and Gmail. As I also admin G.Suite I have access to their paid support. I did your exact follow up - politely and persistent!! It was a dead-end. They had no idea - and worse - no escalation path to the SMTP team.